JP2808857B2 - Heating furnace and manufacturing method of glass preform for optical fiber - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a heating furnace and a method for manufacturing an optical fiber preform, and more particularly, to heat-treating a porous glass preform made of quartz glass fine particles. Furnace and method for heating. The heating furnace of the present invention can prevent the impurity element from being mixed into the glass base material and has excellent durability.

As used herein, the term "heat treatment" refers to heat treatment of a glass preform in a furnace tube which is disposed inside a heating element of a heating furnace and separates the heating atmosphere from the heating element, thereby dehydrating, adding fluorine, and / or heating. Or it means making it transparent.

2. Description of the Related Art As a general method for mass-producing a preform for an optical fiber, a VAD (Vapor Phase Axial Deposition) method is known. In the VAD method, glass particles generated in an oxyhydrogen flame are deposited on a rotating starting member, for example, a glass plate or a glass rod, and a columnar porous base material (soot base material) is formed.
And sintering the porous preform to produce a transparent glass preform for optical fibers.

In order to sinter the porous base material into a transparent glass in the VAD method, the base material needs to be heated to 1600 ° C. or more in an inert gas (for example, helium or argon gas) atmosphere. As a heating furnace used for sintering the base material, a heating furnace having a carbon heating element is usually used. A particular point to be noted in sintering using such a heating furnace is prevention of mixing of transition elements such as copper and iron and mixing of moisture. If the transition element is mixed into the glass preform at 1 ppb or more, the loss wavelength characteristic of the obtained optical fiber is significantly impaired over the entire wavelength. Further, if water is mixed into the base material in an amount of 0.1 ppm or more, the characteristics of the obtained optical fiber in the long wavelength region are impaired.

Therefore, the porous base material is usually dehydrated before or at the same time as the transparency. As a dehydration treatment, a method is known in which a porous base material is heated at a high temperature in an inert gas atmosphere to which a chlorine-based gas and a fluorine-based gas are added. When a fluorine-based gas is used, it has the effect of not only dehydrating the porous base material but also adding fluorine to the glass base material at the same time. The addition of fluorine to the porous preform has the advantage that the refractive index distribution, which is essential for an optical fiber, can be adjusted. This point is described in JP-B-55-15682 and JP-A-55-67533, which will be described later.

The treatment using the fluorine-based gas is usually performed in a heating furnace at the same time as the transparency or as a pre-process. The heating furnace is provided with a furnace tube for isolating the carbon heating element from the sintering atmosphere in order to prevent the carbon heating element from being consumed by moisture or oxygen generated during the heat treatment of the base material. Conventionally, there is a core tube made of quartz glass.

The method using a quartz glass furnace tube is described in JP-B-58-58299, JP-B-58-42136 and JP-A-60-86.
The details are described in each of the publications.

Further, fluorine-based gas is decomposed or reacted at an elevated temperature, F ₂
Generates gas and HF gas. These gases react with quartz glass as shown in the following formula to generate SiF ₄ gas, and the quartz glass is etched by this reaction.

SiO ₂ + 2F ₂ → SiF ₄ + O ₂ SiO ₂ + 4HF → SiF ₄ + 2H ₂ O For this reason, copper and iron existing inside the quartz glass appear on the surface of the quartz glass and become a cause of being mixed into the porous base material. In addition, the etching causes pinholes in the quartz glass furnace tube, which may cause intrusion of outside air and leakage of atmospheric gas to the outside of the furnace, resulting in an adverse effect on the manufacturing process.

In addition, the quartz glass tube has a serious problem that it is easily deformed by high heat. By the way, if it is maintained for a long time even at a temperature of about 1300 ° C., deformation occurs due to viscous flow. In addition, there is a disadvantage that devitrification occurs when used for a long time at 1150 ° C or higher, and once the temperature of the furnace is lowered, a strain derived from a difference in thermal expansion coefficient between the glass layer and the devitrified layer occurs and is destroyed. Had.

By the way, carbon is considered as a material which is hard to react with a fluorine-based gas or a chlorine-based gas. Carbon also does not react with SF ₆ , C ₂ F ₆ , CF _4, etc., which are gases that readily react with quartz. Of course, it does not react with SiF ₄ either.

In fact, At a Japanese Patent Publication No. Sho 56-28852, specific examples but it does not disclose, shows that the carbon muffle tube can be used in the fluorine gas atmosphere, such as F ₂ gas.

However, carbon has the following disadvantages: (1) Since carbon has fine pores, it easily permeates gas. Incidentally, the transmittance of nitrogen for carbon, 10 ⁶ times greater than that of quartz glass.

(2) Carbon is easily oxidized and easily reacts with oxygen at 400 ° C. or higher to form CO ₂ or CO gas.

In order to prevent oxidation, SiC, A
A method of forming a ceramic layer such as l ₂ O ₃ and BN was considered. In fact, although the ceramic layer plays a role of preventing oxidation, there is a problem that it reacts with at least one of a chlorine-based gas and a fluorine-based gas. The impurities generated by this reaction devitrify the soot base material and generate air bubbles in the soot base material.

As described above, carbon is an extremely large gas-permeable material, so that gas enters and exits through the wall, and moisture in the outside air enters through the wall. Therefore, the obtained glass base material contains much water, that is, hydroxyl groups. Cl ₂ , SiF
Conversely, gases such as ₄ are released to the outside of the furnace through the wall, which may deteriorate the working environment. In addition, there is a possibility that impurities (for example, Cu, Fe, etc.) may enter from outside the system. By increasing the thickness of the carbon, these disadvantages can be considerably improved, but not completely.

As described above, there are various difficult problems in adding fluorine to the quartz glass of the clad portion by the conventional method.

[Problems to be Solved by the Invention] In view of the above situation, the present invention solves the problems of the conventional core tube used for dehydration, clarification, and fluorine addition treatment of the optical fiber base material, and has a long life. It is an object of the present invention to provide a long and durable heating furnace for an optical fiber preform having a long life, and to provide a long-life and large-sized heating furnace in which air is prevented from being mixed into a furnace tube.

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the present inventors have found that the inner wall and the outer wall of a furnace tube have a coating of pyrolytic graphite or glassy carbon which has been carbonized in solid phase. It has been found that if a core tube is used, there is no deterioration of the core tube even when a corrosive gas such as a fluorine-based gas or a chlorine-based gas is used at a high temperature. This is because the inner wall is coated with carbon, so that there is no reaction with a fluorine-based gas or a chlorine-based gas, and it has been found that the life is significantly longer than that of a conventional reactor core tube. In addition, since these carbons are dense and have low gas permeability, there is no problem of gas permeation as in conventional carbon furnace tubes.

In other words, the gist of the present invention is a heating furnace in which a porous glass preform for an optical fiber made of a silica-based glass fine particle is heat-treated to obtain a glass preform for an optical fiber, wherein the heating element and the inside of the heating element A furnace tube arranged to isolate a heating atmosphere and a heating element, wherein the furnace tube base material is formed of high-purity carbon, and pyrolytic graphite or solid-phase carbon An optical fiber preform heating furnace characterized by having a coated glassy carbon coating.

In the present invention, the porous glass base material (hereinafter, also referred to as “soot base material”) composed of the silica-based glass fine particles typically includes a soot base material having the following structure.

1. A soot base material or a hollow soot base material in which the entire base material is made of glass particles. In the former case, after the soot base material is made transparent, a hole is made in the center, and a glass rod serving as a core is inserted therein, and finally a glass base material is manufactured.

2. A soot base material in which fine glass particles are deposited around a glass core.

3. A soot base material in which a glass layer that becomes a part of the clad is formed in advance around the glass core and glass fine particles are deposited.

The pyrolytic graphite and the solid carbonized vitreous carbon used for coating the inner surface and the outer surface of the furnace core tube of the heating furnace of the present invention respectively mean the following.

Pyrolytic carbon (Pyrolytic Carbon) is a high temperature (700-1
(200 ° C.) means a graphite obtained by thermally decomposing a raw material mainly composed of hydrocarbons and depositing (depositing) it on a substrate.
There are various methods for coating the furnace core tube substrate as in the present invention, for example, heating the stationary core tube substrate by direct energization, or indirectly heating from the outer periphery by propane, methane, etc. A method in which a hydrocarbon gas is thermally decomposed and deposited is applicable.

Solidified carbonized glassy carbon means glassy carbon obtained by curing and carbonizing a thermosetting resin extremely slowly.Specifically, for example, thermosetting of a phenol resin, a furan resin, etc. It can be obtained by carbonizing a reactive resin or a vinyl chloride resin, but can also be obtained by carbonizing sugar, cellulose, polyvinylidene chloride, or the like. In order to coat the furnace core tube substrate as in the present invention, for example, after thermosetting resin is processed by molding or multiple coating and placed on the furnace core tube substrate, it is very slowly carbonized. Method can be applied.

The core tube material to be coated is preferably high-purity carbon, and particularly preferably a high-purity carbon substrate as described below. The thickness of the coating can be appropriately selected according to the processing conditions.

The purity of the carbon used in the furnace core tube base material is preferably high, generally such that the total ash content is 50 ppm or less, preferably 20 ppm or less. For example, carbon having a total ash content of 1000 ppm is not very suitable for use as the core tube material of the present invention in view of impurities such as iron and copper. Total ash content
Impurities contained in carbon of 20 ppm or less and their amounts are as shown in the following table.

As a dehydrating agent for the glass fine particles, a chlorine compound gas is generally used.To dehydrate the base material using the heating furnace of the present invention, a chlorine-based gas containing no oxygen, for example, Cl ₂ , It is necessary to select SiCl ₄ , CCl ₄ or the like. Chlorine-based gas containing oxygen, for example SOCl ₂ or the like to decompose at a high temperature of dehydration, oxygen is generated, since the oxidation deterioration of the furnace core tube as a main component carbon undesirable. However, CCl ₄ is thermally decomposed to generate carbon powder. Further, Cl ₂ is considered to generate a small amount of oxygen by a reaction of Cl ₂ + H ₂ O → 2HCl + (1/2) O ₂ with water in the porous base material. On the other hand, SiCl ₄ is porous also react with moisture base material without generation of oxygen by the reaction of _{SiCl 4 + 2H 2 O → SiO} 2 + 4HCl, also using a heating furnace of the present invention because even carbon powder does not occur Is the most preferred dehydrating agent. The dehydration temperature is usually 800 to 1
About 200 ° C., and the chlorine-based gas described above is preferably 0.1 to
The dehydration treatment is performed under an atmosphere of an inert gas (for example, argon or helium) containing 10 mol%.

Generally, a fluorine compound gas is used as a fluorine additive to the glass base material. In a quartz furnace core tube, there is a problem that a fluorine compound is etched by fluorine generated by decomposition at a high temperature. Therefore, it is necessary to select a fluorine additive that does not etch the furnace core tube.

However, since the heating furnace of the present invention does not react with fluorine, the range of choice of the fluorine additive is widened, and silicon fluoride, carbon fluoride, or the like can be used. As the silicon fluoride, for example, SiF ₄ , Si ₂ F _{6 and the} like can be used, and as the carbon fluoride, for example, CF ₄ , C ₂ F ₆ , C ₃ F ₈ , CCl ₂ F ₂ , C ₂ Cl ₃
The F ₃ or the like can be used.

In particular, SiF ₄ is toxic and expensive, but is advantageous because it is most commonly available and readily available. In addition, carbon fluoride is advantageous because it is inexpensive and safe, and can be easily handled. In particular, CF ₄ is a particularly suitable fluorine additive when the furnace core tube of the present invention is used because no carbon powder is generated.

The heating temperature during the fluorine addition treatment is about 1100 to 1600 ° C., and the above-mentioned fluorine compound gas is preferably 0.1 to 1
The fluorine addition treatment is performed in an inert gas (eg, argon or helium) atmosphere containing 00 mol%.

In order to carry out transparent vitrification of the base material using the heating furnace of the present invention, it is preferable to carry out the heating at a heating temperature of 1400 to 1600 ° C. while supplying an inert gas (eg, helium, nitrogen, argon, etc.). .

Here, an experiment and a concept on which the present invention is based will be described. Needless to say, the concept described below is
This was explained for the first time based on the knowledge obtained from experiments effective for the present invention, and could not be easily analogized in advance.

Examination of heat resistance Experiment 1: A quartz glass core tube with an inner diameter of 100 mm, a length of 300 mm, and a thickness of 2 mm was heated at 1500 ° C and kept at this temperature all day and night, and the core tube expanded to 400 mm in length. Oops.

Experiment 2: Experiment 1 in which the inner and outer surfaces were coated with a 30 μm thick pyrolytic graphite layer
When a high-purity carbon-based core tube having the same dimensions as that of the above-mentioned core tube was used and heated in the same manner as in Experiment 1, the elongation of the core tube was not observed at all. The same applies to the case where the inner and outer surfaces are coated with 10 μm thick solid carbonized glassy carbon.

In this experiment, a hydrocarbon gas was introduced into the atmosphere at 1000 ° C to coat the pyrolytic graphite layer on the substrate, and the vinyl chloride resin was cured and carbonized to coat the glassy carbon on the substrate. did.

In the following experiments and examples, coating was performed by the above method unless otherwise specified.

Experiment 3: The same furnace tube as in Experiment 1 was heated from room temperature to 1500 ° C every day.
After repeated tests of increasing the temperature over time and further decreasing the temperature from 1500 ° C. to room temperature, the core tube was destroyed by devitrification after 20 days.

Experiment 4: The same heating test as in Experiment 3 was performed on the same core tube as in Experiment 2, and there was no problem after 20 days.

Examination of Oxidation Resistance Experiment 5: No oxidation was observed when the same furnace core tube as in Experiment 2 was used, and the furnace tube was heated to 500 ° C. for 1 hour in the atmosphere.

Experiment 6: When heating the high-purity carbon furnace tube under the same conditions as in Experiment 5 without coating the surface with pyrolytic graphite or glassy carbon, the carbon of the base material was oxidized and the thickness of 50 μm was consumed. .

Examination of Corrosion Resistance Experiment 7: A 30 μm thick pyrolytic graphite was coated on the inner and outer surfaces of a high purity carbon furnace core tube having the same dimensions as in Experiment 1. Cl ₂ / He inside the furnace tube
= 5 mol% / 95 mol% and heated to 1050 ° C., no corrosion of the furnace tube was observed. Also,
No leakage of Cl ₂ gas through the furnace core tube wall was observed.

Similarly, a heating experiment was performed on a high-purity carbon furnace core tube having a 10 μm-thick glassy carbon coating on the inner and outer surfaces. No corrosion of the furnace core tube and no leakage of Cl ₂ gas were observed.

It is considered that the reason why the leakage of the Cl ₂ gas is suppressed is that the coating of the pyrolytic graphite and the glassy carbon is dense.

Experiment 8: An experiment was performed in the same manner as in Experiment 7 using a high-purity carbon core tube having no coating of pyrolytic graphite or glassy carbon, and leakage of Cl ₂ gas through the core tube wall was observed. .

Experiment 9: Experiment 7 was performed using a furnace tube coated with silicon carbide instead of coating the inner and outer surfaces of the furnace tube with pyrolytic graphite or glassy carbon, and the silicon carbide coating on the inner and outer surfaces reacted and volatilized. Moreover, leakage of Cl ₂ gas through the furnace core tube wall was observed.

This reaction (decomposition of SiC) is considered to be based on the following equation: SiC + 2Cl ₂ → SiCl ₄ + C Experiment 10: Using the same furnace core tube as used in Experiment 2, the inside of the furnace core tube was SiF ₄ / When the atmosphere was heated to 1400 ° C. in an atmosphere of He = 3 mol% / 97 mol%, no corrosion in the furnace core tube was observed, and no leakage of SiF ₄ gas through the furnace core tube wall was observed.

Experiment 11: When Experiment 10 was repeated using the furnace core tube having the inner and outer surfaces coated with SiC used in Experiment 9, the coating on the inner and outer surfaces reacted and volatilized.

Therefore, such a furnace core tube has a problem in long-term use.

 From the above Experiments 1 to 11, the following became clear.

i) A furnace core tube made of high-purity carbon as a base material and having inner and outer surfaces coated with pyrolytic graphite or glassy carbon can withstand extremely high temperatures as compared with a furnace tube made of pure quartz glass.

ii) Regarding oxidation resistance, a furnace core tube coated with pyrolytic graphite or glassy carbon has extremely excellent oxidation resistance with respect to a conventional carbon substrate.

iii) Furthermore, even when Cl ₂ or fluorine-based gas is used, the furnace core tube of the present invention has corrosion resistance. In addition, the quartz furnace core tube has difficulty in fluorine gas resistance (SiO ₂ is etched by SiF ₄ (1100 ° C. 1.6 μm / hour)), and a high purity carbon furnace tube coated with SiC. Has difficulty in both chlorine gas resistance and fluorine gas resistance.

Next, a particularly preferred embodiment of the heating furnace of the present invention will be described in more detail with reference to the drawings.

 FIG. 1 shows a heating furnace according to a first embodiment of the present invention.

FIG. 1 is a schematic sectional view of a heating furnace according to a first embodiment of the present invention.

In FIG. 1, 1 is a soot base material, 2 is a support rod, 3 is a furnace tube, 4 is a heating element, 5 is a furnace body, 6 is an inert gas inlet, and 7 is an atmosphere gas (for example, SiF ₄ , helium). Etc.). 31 is a high-purity carbon core tube base material, and 32 is a pyrolytic graphite or glassy carbon coating layer. In the embodiment of FIG. 1, the entire inner and outer surfaces (including the bottom surface) of the furnace core tube are covered.

The soot base material is subjected to heat treatment while traversing the soot base material in the furnace core tube using a heating furnace having a furnace core tube having a coating of pyrolytic graphite or glassy carbon on the entire surface.

In a second aspect of the present invention, the furnace core tube comprises a detachably connected upper, middle and lower portion, at least the center of which is formed from pyrolytic graphite or high purity carbon coated with glassy carbon. The upper and lower portions are formed of a heat and corrosion resistant material.

Hereinafter, the second embodiment will be described in detail based on an example shown in the drawings.

FIG. 2 shows a schematic cross-sectional view of the heating furnace. Heating furnace body 5
A heating element 4 is provided inside the furnace, and a furnace tube 3 is provided at the center of the furnace body.

The core tube 3 comprises an upper portion 34, a central portion 35 and a lower portion 36, each of which is detachably connected by a suitable means, for example, a screw. The core tube central part 35 is formed of a high-purity carbon base material 31 having a coating 32 of pyrolytic graphite or glassy carbon.

The upper part 34 and lower part 36 of the core tube are not as hot as the central part 35, so the surface is coated with SiC or Si ₃ N ₄ which satisfies the requirements of oxidation resistance, corrosion resistance to chlorine-based gas and / or fluorine-based gas. It may be made of high-purity carbon (the upper and lower coatings are not shown for simplicity).
(SiC and Si ₃ N ₄ do not react with Cl ₂ or fluorine at low temperatures.) To apply a coating of SiC or Si ₃ N _{4 to} a substrate, for example, a CVD (Chemical vaper deposition) method can be adopted.

The core tube of the present invention, in which the base material is made of high-purity carbon, is suitable because it does not react with a halogen-based gas at all unless the atmosphere contains oxygen and moisture, and has excellent heat resistance.

However, when processing the porous base material, the carbon in the central portion 35 exposed to high temperatures due to the moisture absorbed in the base material and the moisture and oxygen gas that has entered from outside the system has been used for a long time. May wear out. Further, the carbon inner wall is easily consumed due to a special cause accompanying the processing of the porous base material described below.

That is, the SiO ₂ powder released from the porous base material adheres to the inner wall of the carbon and reacts with the carbon to form SiC. At that time, the generated oxygen further reacts with the carbon to form CO. The formed SiC easily reacts with chlorine gas used during dehydration. In this way, the inner carbon wall is
It reacts with O ₂ powder and is consumed.

These reactions can be represented by the following series of equations.

SiO ₂ + C → SiC + O ₂ O ₂ + 2C → 2CO SiC + Cl ₂ → SiCl ₄ + C Further, Cl ₂ used as a dehydrating agent reacts with moisture in the porous base material: Cl ₂ + H ₂ O → 2HCl + (1/2) O ₂ Oxygen generated by this reaction also causes carbon to be consumed.

Therefore, the carbon material in the central portion needs to be replaced when used for a long time.

On the other hand, the upper and lower portions of the core tube are not exposed to high temperatures as much as the central portion, so they are not consumed much. If the core tube has a three-stage structure as in the present invention, only the worn central portion is replaced. Is preferred.

Further, since carbon is porous, it is necessary to sufficiently remove adsorbed moisture at a high temperature before use. Therefore,
From the viewpoint of removing adsorbed water, it is better that the replacement frequency of the carbon furnace tube is low.

One method of preventing oxidation of the furnace tube is to set the temperature of the glass base material in and out of 500 ° C. or less at which carbon is not oxidized. However, in this method, the operation rate of the furnace is greatly reduced. In addition, the inside of the reactor core tube cannot be prevented from being contaminated with dust in the atmosphere. Such prevention of the air from being mixed into the furnace tube is achieved by the heating furnace according to the third aspect of the present invention. That is, the heating furnace according to the third aspect of the present invention has a front chamber for accommodating the porous glass base material and taking in and out of the furnace tube in addition to the heating element and the furnace tube. Front room is 80
Preferably, it is possible to heat to 0 ° C. and reduce the pressure to 10 ⁻² torr.

The anterior chamber is preferably made of a material that can withstand high temperatures and does not generate impurities, for example, quartz glass, SiC, Si ₃ N ₄ , and BN. The anterior chamber may be made of the same material as the furnace tube, or may be made of a different material.

 Hereinafter, the third embodiment will be described with reference to the accompanying drawings.

FIG. 3 is a schematic sectional view showing an example of a heating furnace according to a third embodiment. This heating furnace is the same as the heating furnace shown in FIG.
11 (note that the upper and lower coatings are shown in FIG. 3), in addition to the heating furnace shown in FIG. 2, a front chamber 11, a front chamber gas outlet 14, and a front chamber purge gas inlet.
15 and a partition 16 are provided.

In order to insert the porous glass body into the heating furnace of FIG.

1. Close partition 16. (Initial state, open during soot processing.) 2. Mount porous glass preform 1 via support rod 2 to chuck that can rotate and move up and down.

3. Open the upper lid of the front chamber 11 and lower the porous glass preform 1 into the front chamber 11.

4. Close the lid, replacing the previous chamber with an inert gas (N ₂ or He, etc.).

5. Open the partition 16 separating the heating chamber and the front chamber 11, and introduce the porous glass preform 1 into the heating atmosphere previously maintained at the heating temperature.

Further, to take out the base material from the heating furnace of the present invention, the following procedure is performed.

1. The base material 1 after the heat treatment is pulled up from the heating atmosphere to the front room 11. At that time, the temperature of the heating atmosphere does not necessarily need to be lowered.

2. Close partition 16.

3. Open the upper lid of the front room 11 and take out the base material 1.

Another gist of the present invention is to provide a porous glass preform made of quartz-based glass fine particles in the above-mentioned heating furnace, that is, a furnace tube base material made of high-purity carbon, and an inner surface and an outer surface thereof. A glass preform for optical fiber characterized by having a coating of pyrolytic graphite or solid carbonized glassy carbon on a heating furnace, if necessary after dehydration, and then silicon fluoride and carbon fluoride as fluorine additives. A glass preform for an optical fiber, comprising adding fluorine and simultaneously or subsequently clarifying the glass microparticles by heat treatment in an inert gas atmosphere containing at least one fluoride selected from fluorides. In the manufacturing method.

Contamination or during processing of the muffle tube, to remove completely the adsorbed dust and moisture, prior to use, a chlorine-based gas, in particular at a temperature of 1500 ° C. or higher in an atmosphere containing Cl _2, several hours bakeout the core tube It is desirable to do. In an optical fiber manufactured from a glass base material obtained by adding fluorine in a furnace tube that is not fired, significant absorption due to moisture and impurities may be observed.

In each step of the method of the present invention, the conditions and the type of gas used described in relation to the heating furnace of the present invention can be adopted.

As the fluorine dopant used in the method of the present invention, those described above can be used, and among them, SiF ₄ is most suitable. SiF ₄ is preferably a high-purity product of 3N or more.

Although SiF ₄ does not react with carbon at all, if the soot base material is used without being sufficiently dried, smoke may be generated in the carbon furnace tube when fluorine is added. This is considered to be caused by the moisture in the soot base material reacting with SiF ₄ and carbon. As a result, deposits like carbon particles may be deposited on the upper part of the soot base material. In order to prevent this, it is preferable to dehydrate and dry the soot base material before subjecting the soot base material to heat treatment in a carbon furnace tube in an atmosphere to which SiF ₄ is added.

Of course, the dehydration can be performed simultaneously with the addition of fluorine as described above, but it is preferable to perform the dehydration prior to the addition of fluorine from the above reasons and the point of the dehydration effect.

The addition of fluorine to the soot matrix by SiF ₄ can be efficiently performed at a temperature of 1000 ° C. or higher, preferably 1100 to 1400 ° C. Fluoridation must be performed well before the soot matrix shrinkage is complete.
When shrinkage occurs in a state in which fluorine is not sufficiently added, fluorine is not added to the entire soot base material, fluorine is added non-uniformly, and a distribution of the amount of added fluorine occurs.

As described above, the soot base material is manufactured by the flame hydrolysis method, and is made of glass fine particles having a particle size of 0.1 to 0.2 μm.

 Hereinafter, the method of the present invention will be described in more detail.

Preparation of Soot Base Material In order to generate quartz glass fine particles by a flame hydrolysis reaction, as shown in FIG. 4 (a), a quartz concentric multi-tube burner 41 is used to convert oxygen, hydrogen and raw material gas. SiCl ₄ or SiCl ₄ and a dopant compound (for example,
A mixed gas with GeCl ₄ ) may be sent to the center of an oxyhydrogen flame using an inert gas (eg, argon or helium) as a carrier gas to cause a reaction.

An inert gas is supplied for shielding so that the source gas reacts in a space several mm away from the tip of the burner 41. When obtaining a soot base material rod, glass particles are deposited axially from the tip of the rotating seed rod 46. In addition, when obtaining a pipe-shaped soot base material, FIG. 4 (b)
As shown in FIG. 7, after the glass particles are laminated while the burner 41 is traversed on the outer periphery of the rotating quartz rod or carbon rod 46, the center member 46 is removed. In addition, 46 may be a core glass rod, and in this case, it is not necessary to pull out the center member. Further, a plurality of burners 41 may be used.

Addition of fluorine to soot base material and clarification (sintering) The soot base material obtained by the above method is made of, for example, high-purity carbon whose inner and outer peripheral portions are coated with a material having low gas permeability as shown in FIG. In the furnace tube (upper and lower flanges and cylindrical muffle), the furnace is made to stand by at a position corresponding to the upper part of the heating element, the inside of the furnace tube is made to be a helium atmosphere containing Cl ₂ gas, and the temperature of the heating element is raised to 1050 ° C. From this point, the soot base material is lowered at a speed of about 2 to 10 mm / min. After the entire soot base material has passed the side of the heating element, the descent of the soot base material is stopped, then the supply of Cl ₂ gas is stopped, and instead, a helium atmosphere containing SiF ₄ is used. When the temperature reaches ° C, fluorine is added while raising the soot base material at a speed of 4 mm / min.

Again, after all the soot has passed the side of the heating element, the SiF ₄
Turn off the gas supply and set the atmosphere to He only.
When the temperature reaches 1600 ° C, the soot base material is clarified while descending at a speed of 3 mm / min.

Next, the present invention will be described more specifically with reference to examples.

Example 1 Using the heating furnace shown in FIG. 2, the porous glass base material was subjected to dehydration, fluorine addition, and transparency treatment.

The furnace core tube has an inner diameter of 200 mm, a thickness of 10 mm, and a length of 1000 mm.A high-purity carbon substrate with pyrolytic graphite 30 μm thick on the inner and outer surfaces and a 50 μm thick SiC on the inner and outer surfaces Consists of the top and bottom of the lumber.

The processing conditions are as follows: (dehydration treatment) 1050 ° C, Cl ₂ / He = 5 mol% / 95 mol% Base material descent rate 5 mm / min (fluorine addition treatment) 1370 ° C, SiF ₄ / He = 3 mol% / 97 mol%, base material rising speed 3 mm / min (clearing treatment) 1600 ° C., He 100%, base material descending speed 5 mm / min The relative refractive index difference Δn of the obtained transparent glass base material was 0.34%. However, [In the formula, n represents a refractive index, a subscript SiO ₂ represents pure quartz, and F represents a fluorine-doped base material. Using the obtained base material as a clad, a pure silica core single mode fiber was produced. When the transmission loss of this fiber was measured, the loss was 0.17 dB / km at an optical wavelength of 1.55 μm, and no absorption due to the presence of Cu and Fe was observed at all.

Using the same furnace core tube, a fluorine-containing glass
Heat treatment was performed on 50 tubes, but no deterioration of the furnace core tube was observed.

Example 2 As shown in FIG. 6, outside the furnace core tube 3 of Example 1,
A high-purity carbon extracorporeal tube 10 (coated on the entire inner and outer surfaces of the extracorporeal tube, having a thickness of 30 μm) coated with pyrolytic graphite was provided to form a double structure. (For simplicity, the coating of the extracorporeal tube is not shown.) Each process was performed under the same conditions as in Example 1. In this embodiment, He gas was supplied from the inlet 8 at a rate of 10 / min between the furnace core tube and the outer tube so that the pressure became negative with respect to the furnace core tube internal pressure.

A pure silica core single mode fiber was manufactured using the base material obtained in this example as a clad. The transmission loss of this fiber at an optical wavelength of 1.55 μm is
It was 0.17 dB / km and the residual moisture was less than 0.01 ppm.

In the heating furnace used in the present embodiment, even the slight leakage from the joint of the furnace core tube, which was observed in the case of the first embodiment, is discharged by the He that supplies the double structure portion, so that the working environment is directly contaminated. There is an advantage that there is no.

An experiment was performed on the same furnace core tube (thickness: 10 μm) coated with solid carbonized glassy carbon in place of the pyrolytic graphite under the same conditions, but equivalent results were obtained.

Example 3 In this example, a furnace core tube coated with pyrolytic graphite on all inner and outer surfaces (thickness 30 μm) or a furnace core tube coated with glassy carbon on all inner and outer surfaces (thickness 10 μm) is shown in FIG. The heating furnace of the embodiment shown was used.

The porous glass body was placed in the front room 11 set at 200 ° C., the upper lid of the front room was closed, nitrogen gas was supplied into the front room at 10 / min for 10 minutes, and the front room was replaced with nitrogen gas. After that, the partition 16 is opened, the porous glass body 1 is moved from the anterior chamber into the furnace core tube, and dehydration, fluorine addition, and transparency processing are performed.
A transparent optical fiber glass preform was manufactured. At the time of taking out the base material, the partition was closed after moving the glass base material to the front room, and then the upper lid was opened and the glass base material was taken out.

The conditions of the respective processes are shown below: (dehydrated) 1050 ℃, SiCl ₄ 200cc / min, He20 / min, the traverse speed of 5 mm / min (fluorine addition treatment) 1270 ℃, SiF ₄ 800cc / min, He20 / min, the traverse Speed 4mm / min (Transparency treatment) 1500 ° C, He20 / min, SiF ₄ 800cc / min, traverse speed 10mm / min Using the obtained base material as a cladding, a pure quartz single mode fiber was manufactured. The transmission loss of this fiber is 1.55
The loss was as low as 0.17 dB / km at μm.

In the case of the heating furnace of the present embodiment, the furnace temperature is 800
Since the base material can be processed without lowering the temperature to below ℃, it is very advantageous from the viewpoint of the productivity of the processed base material.

Comparative Example 1 A fiber was manufactured under the same conditions as in Example 1 except that a quartz glass tube containing 1 ppm of copper and having no carbon layer was used as a quartz glass core tube. The residual moisture of the obtained fiber was 0.01 ppm. In addition, although absorption derived from copper was present up to around 1.30 μm, this value was sufficiently lower than conventional absorption, and the absorption amount was 2-3 at a wavelength of 0.8 μm.
dB / km. However, the inner wall of the furnace tube was significantly etched, which proved to be problematic in terms of corrosion resistance.

Comparative Example 2 (Heat resistance of quartz glass furnace tube) A soot base material was produced by repeating the method of Example 1 except that the quartz glass furnace tube was used. It became impossible to reuse.

Comparative Example 3 (Etching of Quartz Glass Furnace Tube) When SF ₆ was used in Comparative Example 2 instead of SiF ₄ , the quartz glass tube was significantly etched, and pinholes were formed on the furnace wall near the heating element. In addition, a large amount of water of several ppm was present in the obtained glass base material. Of course, the core tube was significantly stretched, and it was impossible to reuse it.

Comparative Example 4 In the heating furnace of Example 1, instead of coating with pyrolytic graphite or glassy carbon, a furnace core tube in which only the outer surface of a high-purity carbon base material was coated with SiC to a thickness of 50 μm was used.

The preform was processed under the same conditions as in Example 1, and a single-mode fiber was similarly produced using the obtained preform. The transmission loss of this fiber is 0.18 dB / km at an optical wavelength of 1.55 μm.
And low loss.

However, by repeatedly using the heating furnace, the Cl ₂ gas diffused in the carbon substrate peeled off the SiC layer and eroded the furnace body, making it impossible to use the furnace. In addition, a very serious problem has arisen in terms of the working environment.

Example 4 In the heating furnace used in Example 1, instead of coating with pyrolytic graphite, a furnace core tube was used in which the inner and outer surfaces of a high-purity carbon base material were coated with 10 μm of glassy carbon that had been solidified into carbon.

 The glass base material was processed under the same processing conditions as in Example 1.

After the start of the preform treatment, the transmission loss of the fiber using the fifth preform is 0.02 to 0.02 at an optical wavelength of 1.55 μm, as compared with the case of using a furnace tube having a pyrolytic graphite coating.
0.03dB / km higher.

This can be considered to be based on the element concentration distribution in the depth direction of each coating shown in FIGS. 7 and 8. FIG. 7 shows the case of coating with pyrolytic graphite, and FIG. 8 shows the case of coating with vitreous carbon solidified into carbon. The graphs shown in FIGS. 7 and 8 are the results of the impurity concentration distribution in the depth direction measured by SIMS (secondary ion mass spectrometry). The vertical axis is the concentration of each element, and the horizontal axis is the depth direction of the coating (the left end is the surface). The vertical axis is a qualitative expression. As is clear from the surface concentration and the shape of the curve shown in the graph, the glassy carbon coating has a higher impurity (alkali metal, transition metal, etc.) concentration than pyrolytic graphite. And the result of this example is considered to be due to this.

However, even in the case of glassy carbon, a low-loss (0.17 dB / km) fiber was obtained after the sixth fiber. This is considered to be due to the disappearance of impurities in the glassy carbon coating.

Example 5 The heat treatment apparatus shown in FIG. 9 was used. In this heat treatment apparatus, a heating element 4 is provided in a hollow ring-shaped furnace main body 5 at a central outer peripheral portion of a cylindrical furnace core tube 3 formed of carbon so as to surround the furnace core tube. The optical fiber preform 1 inserted into the furnace core tube is heated by a heating element.

In the apparatus shown in FIG. 9, the carbon furnace core tube 3 is divided into three parts (34, 35 and 36), and a front chamber 11 is provided to prevent oxidation of the furnace core tube. A gas-impermeable pyrolytic graphite film 32 is coated on the inner surface and the outer surface of the central portion 35 of the carbon core tube with a thickness of 30 to 40 μm. Further, since the upper part 34 and the lower part 36 of the furnace core tube do not become as hot as the central part 35, a carbon furnace core tube having a SiC film 37 on the inner and outer surfaces that satisfies oxidation resistance and corrosion resistance to chlorine-based gas and fluorine-based gas. It was used. Treated porous matrix 1
Was 500 mm in length and 140 mm in diameter, and was manufactured by the VAD method.

The method of using the heating furnace of FIG. 9 is substantially the same as the case of using the heating furnace of FIG. That is, first, the partition plate 16
The base material 1 is put in the front room 11 in a state where the is closed, and the upper lid 8 is closed. Nitrogen gas is supplied at a rate of 10 / min into the front chamber from the gas inlet 15 to replace the front chamber with nitrogen gas. Then the divider
16 is opened, and the porous preform is moved from the anterior chamber 11 into the furnace core tube and subjected to a heat treatment to obtain a transparent glass preform for optical fiber. When taking out the base material, the glass base material is moved to the front room, the partition plate 16 is closed, and then the upper lid is opened to take out the base material.

(Manufacture of core part) After moving the porous base material from the anterior chamber into the furnace core tube, the inside of the furnace core tube was heated to 1050 ° C, and the porous material was supplied while supplying He20 / min and SiCl ₄ 200 cc / min. The glass base material was lowered at 5 mm / min to remove moisture and impurities in the base material. After the base material passed the side of the heating element, only He20 / min was supplied, and the temperature in the furnace core tube was raised to 1550 ° C., and the temperature was raised at 4 mm / min to perform vitrification.

(Manufacture of the clad part) As in the case of the manufacture of the core part, SiCl ₄ 200 cc / min.
The base material was dehydrated by supplying 0 / min. Thereafter, the furnace core tube temperature was increased to 1200 ° C., CF _{4 was} supplied at 900 cc / min and He was supplied at 20 / min, and the base material was raised at 3 mm / min to perform a fluorine addition treatment. Subsequently, the temperature in the furnace core tube was raised to 1500 ° C, and He20 /
While supplying the amount, the base material was lowered at 4 mm / min to form a transparent glass.

The obtained glass base material contains about 1.3% by weight of fluorine,
The value under the relative refractive index (Δn) was 0.35%.

(Manufacture of Optical Fiber) The pure quartz preform obtained in the manufacture of the core portion was stretched to a diameter of 5 mm in an electric resistance furnace. Further, a hole having a diameter of 5 mm was made in the center of the fluorine-containing glass base material obtained in the production of the clad portion, and dummy pipes were connected to both ends.
Attaching the fluorine-containing glass preform in an electric furnace, and smoothed by etching with SF ₆ while maintaining the pipe inner surface to a high temperature. Thereafter, a pure quartz preform stretched to 5 mm was inserted into the pipe, and both were heated and integrated in a Cl ₂ atmosphere to obtain an optical fiber preform.

After stretching the integrated optical fiber preform to an outer diameter of 35 mm, it was drawn to a core diameter of 10.5 μm and a cladding diameter of 12 mm.
A 5 μm pure silica core single mode optical fiber was obtained. When the transmission loss of the optical fiber was measured, the wavelength was 1.55 μm.
At 0.18 dB / km.

Example 6 A rod (18 mm in outer diameter) composed of quartz glass having a center portion containing 6% by weight of GeO ₂ and an outer peripheral portion made of pure quartz glass was synthesized by a VAD method, and a porous preform was further formed on the outer periphery of the rod by a VAD method. It was deposited by the method. The outer diameter of the porous base material at this time is 140 mm
Met.

This base material was subjected to dehydration and transparency treatment under the same conditions as in Example 5. That is, after the porous base material was placed in the front chamber and the front chamber was replaced with nitrogen gas, the base material was moved into the furnace core tube. The furnace core tube temperature is 1050 ° C, He20 / min, SiCl ₄ 200
While supplying cc / min, the base material was lowered at 5 mm / min to dehydrate the base material. When the base material passed by the side of the heating element, the temperature in the furnace core tube was raised to 1500 ° C., and the base material was raised at 4 mm / min while supplying He 20 / min to form a transparent glass.

The outer diameter of the transparent glass base material for optical fibers was 65 mm, and the ratio of the outer diameter to the GeO ₂ core portion was 15.0. After stretching the base material to an outer diameter of 35 mm, it was drawn into an optical fiber having a diameter of 125 μm.

The transmission loss of the obtained optical fiber was excellent at 0.35 dB / km at a wavelength of 1.3 μm and 0.20 dB / km at a wavelength of 1.55 μm, and the initial tensile strength was at a level of 5.5 kg, which was no problem.

After heat treatment was performed on 200 porous base materials, the inner surface of the carbon furnace core tube was observed.The pyrolytic graphite coating on the inner surface of the center portion of the furnace core tube was slightly oxidized, resulting in deformation and deterioration. There was no problem.

[Effects of the Invention] The present invention can produce an optical fiber preform free of impurities, particularly iron, copper and moisture, by reducing the consumption of a furnace tube, and can produce light with low transmission loss from the produced glass preform. Fiber can be obtained.

By coating the inner and outer surfaces of the high-purity carbon core tube base with pyrolytic graphite or solid carbonized glassy carbon, even if the core tube is used at high heat, it will be thermally consumed and consumed by corrosive gas. Less, and excellent durability,
It is also economically advantageous.

Furthermore, by using a high-purity carbon base material, there is no risk of contaminating the porous base material with impurities, and it does not react with fluorine-based gases (CF ₄ , SF ₆ , SiF _4, etc.) The durability can be further improved such that there is no fear of breakage even at a high temperature, for example, 1800 ° C. or higher.

By providing the heating chamber with the front chamber, the atmosphere (atmosphere of the working chamber) is not mixed into the heating atmosphere, and contamination by impurities in the furnace core tube is eliminated. Therefore, the devitrification of the base material can be prevented, and the transparency is improved. Since the furnace body is not cooled when the glass body is taken in and out, the operating rate of the furnace is high. If the furnace core tube is made of pyrolytic graphite or carbon having a coating of vitreous carbon that has been solidified into carbon, the carbon is less likely to be oxidized, so that the life of the furnace core tube is extended and the inside of the furnace core tube is reduced. The graphite particles are not suspended, and the low-strength portion of the optical fiber made of the glass base material is reduced.

Further, when a carbon furnace core tube is used as in the heating furnace of the present invention, there is no problem of deformation due to heat, deterioration due to crystallization, destruction, etc., which is a problem when using a furnace core tube made of quartz. The furnace core tube can be used for a long time. Also, when using a quartz material, it was difficult to process a large-diameter furnace core tube, but when using a carbon material, a furnace core tube with a larger diameter than when using quartz can be used, so that a base material that can be processed is used. This is also advantageous from the viewpoint of increasing the size.

In the above embodiment, the case where a zone furnace is used has been described as an example. However, the same effect can be obtained even when a soaking furnace is used.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of a heating furnace for an optical fiber preform according to the first embodiment of the present invention, and FIG. 2 is a diagram showing heating of the optical fiber preform according to the second embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of a heating furnace for a preform for an optical fiber according to a third embodiment of the present invention, and FIGS. 4 (a) and 4 (b) are flame flame hydrolysis methods. ,
FIG. 5 is a diagram illustrating a method of manufacturing a soot preform, FIG. 5 is a diagram illustrating a refractive index distribution of a single mode fiber obtained by the method of the present invention, and FIG. 7 and 8 are graphs showing the distribution of the impurity concentration in the coating depth direction, and FIG. 9 is a schematic view of the heating furnace used in Examples 5 and 6 of the present invention. It is an outline sectional view. DESCRIPTION OF SYMBOLS 1 ... Soot base material, 2 ... Support rod, 3 ... Furnace tube, 4 ... Heating element, 5 ... Furnace body, 6 ... Inert gas inlet, 7 ... Atmospheric gas inlet, 8 ... ... supply port, 10 ... external tube, 11 ... front chamber, 12 ... heating element, 14 ... front chamber gas outlet, 15 ... front chamber purge gas inlet, 16 ... partition 31 ... high-purity carbon core tube Base material, 32: Pyrolytic graphite or glassy carbon coating, 33: Carbon layer, 34: Upper core tube, 35: Central core tube, 36: Lower core tube, 37: SiC film, 41 …… Multi-tube burner, 46 …… Seed rod.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ichiro Tsuchiya 1st Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Inside Sumitomo Electric Industries, Ltd. (72) Inventor Hiroshi Yokota 1st Tayacho, Sakae-ku, Yokohama-shi, Kanagawa (56) References JP-A-2-196045 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C03B 37/00-37/16

Claims (3)

(57) [Claims] 1. A heating furnace for heat-treating a porous glass preform for an optical fiber composed of fine silica-based glass particles to form a glass preform for an optical fiber, wherein the heating furnace is disposed inside the heating element. A furnace tube for isolating the heating atmosphere and the heating element, wherein the furnace tube base material is formed of high-purity carbon, and the inner surface and the outer surface thereof are pyrolytic graphite or vitreous carbon solidified into carbon; In a furnace for heating a glass preform for optical fibers, characterized by having a coating of, a porous glass preform made of silica-based glass fine particles is made of Cl ₂ , CCl
₄ and a dehydration treatment using at least one chloride selected from SiCl ₄ as a dehydrating agent, and then a gas containing at least one fluoride selected from silicon fluoride and carbon fluoride as a fluorine additive A method for producing a glass preform for an optical fiber, comprising: performing fluorine addition treatment in an atmosphere; and simultaneously or thereafter, making the glass fine particles transparent. 2. A heating furnace for heating a porous glass preform for optical fiber comprising a silica-based glass fine particle body into a glass preform for optical fiber, wherein the heating furnace is disposed inside the heating element. A furnace tube for isolating the heating atmosphere and the heating element, wherein the furnace tube base material is formed of high-purity carbon, and the inner surface and the outer surface thereof are pyrolytic graphite or vitreous carbon solidified into solid carbon; In a heating furnace for an optical fiber glass preform characterized by having a coating, a porous glass preform made of silica-based glass particles is subjected to a dehydration treatment using SiCl ₄ as a dehydrating agent, and simultaneously or Then, a method of manufacturing a glass preform for an optical fiber, which comprises making the glass fine particles transparent. 3. A heating furnace for heat-treating a porous glass preform for an optical fiber comprising fine silica-based glass particles to form a glass preform for an optical fiber, wherein the heating furnace is disposed inside the heating element. A furnace tube for isolating the heating atmosphere and the heating element, wherein the furnace tube base material is formed of high-purity carbon, and the inner surface and the outer surface thereof are pyrolytic graphite or vitreous carbon solidified into solid carbon; oven optical fiber glass preform, characterized in that it comprises a coating, the porous glass preform made of silica based glass particulate matter, the fluorine addition treatment in a gas atmosphere containing CF ₄ as the fluorine additive A method for producing a glass preform for an optical fiber, which comprises carrying out and simultaneously or thereafter, making the glass fine particles transparent.